The present invention relates to an improvement in error-diffusion-based halftone techniques usable in a digital printer, such as a xerographic or ink-jet digital printer.
Common systems for printing an image based on original digital image data, such as xerographic printing and the various forms of ink-jet printing, are typically xe2x80x9cbinaryxe2x80x9d or xe2x80x9call or nothingxe2x80x9d printing processes. In other words, for a pixel-sized small area in an image to be printed, the nature of the printing hardware is such that the hardware can place a mark in the area, rendering the small area completely black, or not print in the area, rendering the small area white. However, image data is readily available as grayscale data: that is, for any pixel-sized area in an image to be printed, the image data can express a grayscale level for the individual pixel area typically on a scale of 0 (completely white) to 255 (completely black), with each of the intermediate levels representative of a different gradation of gray. (Of course, although a monochrome binary system will be described throughout this specification, the general principles will of course apply to color and multi-level printing as well.) A common design challenge is attempting to render a grayscale area over a multipixel area using such a binary digital printing apparatus. Over a large number of pixel areas, one can distribute black pixels so that, viewed from a distance, the area simulates a grayscale of a desired darkness.
One well-known technique for simulating a gray area with a distribution of black pixels is known as error diffusion. In one straightforward embodiment of error diffusion, for each pixel, the apparatus will decide to print the pixel black if the grayscale is above a certain threshold, such as 128 on the 0 to 255 scale, and leave the area white if the grayscale is below the threshold. However, when each decision is made, the error resulting from the decision, that is the difference between the actual grayscale of the pixel datum and the threshold or some other number, is xe2x80x9cdiffusedxe2x80x9d to a set of pixels which neighbor the pixel in question. By diffusing the error, a certain amount of grayscale is either added or subtracted to the grayscale of the neighboring pixel, and will thus influence whether that neighboring pixel will be printed black or white. Over a large number of pixels, the desired overall effect is that a gray area will be created of a desired darkness.
One advantage of error diffusion is that the calculations can be made in real time, essentially while the image is being printed: for a running set of pixels, the accumulated errors associated with each of a series of pixels, which determine whether the particular pixel area should be printed black or white, are assigned to buffers in a memory, and these buffers accumulate errors from various neighboring pixels up until the time a particular pixel is about to be printed.
Although error diffusion techniques are largely successful at simulating grayscales with digital printing devices, any number of subtle practical problems have been observed with the basic technique. One noticeable problem with basic error diffusion techniques is xe2x80x9cwormsxe2x80x9d. In general, these print defects result from the fact that, when a grayscale area is simulated with error diffusion, those black spots which are generated by the error diffusion algorithm have a tendency not to be evenly distributed through the intended grayscale area, but rather aggregate in curved lines within the area, thus forming conspicuous print defects. Various techniques have been attempted in the past for overcoming these defects, while still maintaining the simplicity of the error diffusion technique.
The present invention relates to an improvement in the basic error diffusion printing system which could be used with any digital printer, such as a xerographic printer or any type of ink-jet printer.
U.S. Pat. No. 5,535,019 is representative of currently-known techniques to improve error diffusion half toning. Columns 1-2 of the patent give an overall treatise on the prior art of error diffusion printing, with references to further art.
U.S. Pat. 5,748,785 discloses an advanced system of error diffusion, which can be used in conjunction with the present invention. This advanced type of error diffusion system is particularly useful in printing full-color images.
According to one aspect of the present invention, there is provided a method of processing digital image data, for printing with a digital printing apparatus. A digital image datum corresponding to a pixel is submitted to a function whereby, for a given pixel in the image data, the digital printing apparatus is instructed to print a mark if the image datum for the given pixel is above a first predetermined threshold, and an error derived from a relationship between the datum and the threshold is distributed among neighboring image data of a predetermined spatial relationship to the given pixel. For a digital image datum having a grayscale value below a second predetermined threshold, an artificial image datum is substituted for the image datum.
According to another aspect of the present invention, there is provided a method of processing digital image data, for printing with a digital printing apparatus, the digital printing apparatus being capable of outputting a mark in a pixel area of a predetermined grayscale value. A digital image datum corresponding to a pixel to a is submitted to a function whereby, for a given pixel in the image data, the digital printing apparatus is instructed to print a mark of the predetermined grayscale value if the image datum for the given pixel is within a first predetermined grayscale range, and an error derived from a relationship between the datum and the grayscale range is distributed among neighboring image data of a predetermined spatial relationship to the given pixel. For a digital image datum having a grayscale value within a second predetermined grayscale range, an artificial image datum is substituted for the image datum.